dart2js cps: Translate identity placeholders to refinement nodes.

pkg/compiler/lib/src/js/js.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library js;
 
 import 'package:js_ast/js_ast.dart';
 export 'package:js_ast/js_ast.dart';
 
 import '../common.dart';
@@ skipping 164 matching lines @@
       }
       _cachedLiteral = js.escapedString(text);
     }
     return _cachedLiteral;
   }
 
   @override
   String get value => _literal.value;
 }
 
+/// True if the given template consists of just a placeholder. Such templates
+/// are sometimes used to manually promote the type of an expression.
+bool isIdentityTemplate(Template template) {
+  return template.ast is InterpolatedExpression;
+}
+
 /// Returns `true` if [template] will immediately give a TypeError if the first
 /// placeholder is `null` or `undefined`.
 bool isNullGuardOnFirstArgument(Template template) {
   // We look for a template of the form
   //
   //     #.something
   //     #.something()
   //
   Node node = template.ast;
   if (node is Call) {
     Call call = node;
     node = call.target;
   }
   if (node is PropertyAccess) {
     PropertyAccess access = node;
     if (access.receiver is InterpolatedExpression) {
       InterpolatedExpression hole = access.receiver;
       return hole.isPositional && hole.nameOrPosition == 0;
     }
   }
   return false;
 }